This invention relates generally to ethylene polymerization, and more specifically to methods and apparatus for use of chromium-based catalysts for the production of polyethylene in a gas phase polymerization reactor, and in particular, chromium based catalysts which are chromium oxide based.
In gas phase polymerization reactions, it is generally accepted that slurry catalyst feed is more reliable and easily controlled than dry catalyst feed. From a mechanical standpoint, slurry feeders are much simpler than dry feeders. Feeding and precisely metering a fluid (the catalyst slurry) is simpler than attempting to convey a solid catalyst stream into the polymerization reactor at high differential pressure with precision control of feed rates. Thus, a solid catalyst feeder is more complicated in design and maintenance. Slurry feeders, on the other hand, are simpler in design and provide a positive means of controlling and measuring the feed rate of catalyst into the polymerization reactor. This translates into lower maintenance costs and less downtime. Furthermore, the precise control of catalyst feed rate available with slurry feeders helps mitigate the risk of runaway reactions and sheeting in these polymerization reactors. Also, if dry catalyst feeders could be eliminated from the reactor design package, the start-up cost for a new plant would be substantially reduced.
Slurry catalysts are used in other polymerization processes, notably the “Phillips Slurry Loop” type process, however these catalysts are fed as concentrated “muds” in the polymerization solvent, typically in “shots”. This type of catalyst feed is not useful in Gas Phase polymerizations due to the large sizes of these shots and the difficulty encountered in dispersing such a large amount of catalyst within the gas phase polymerization reactor.
Despite this utility, however, with chromium-based catalysts, such as supported silylchromate and chromium oxide catalysts, slurry catalyst feed was not pursued. Due to the specific nature of the gas phase polymerization process, the catalyst must remain in suspension in the slurry solvent without significant settling, without agitation, for periods of 5 minutes to 1 hour. Since solvents used in “mud” feeding in Slurry Polymerization reactors are normally light hydrocarbons, such as isobutane, hexane, or isopentane, these solvents would not meet this criteria. Additionally, certain chromium based catalysts contain Cr+6 which can chemically oxidize slurry solvents, resulting in changes in catalyst performance over time.
Because of these requirements, the use of a higher viscosity slurry solvent, typically a mineral oil, has been required in practice. It was also believed that either impurities in the mineral oil, the typical slurry solvent used for catalyst feed to Gas Phase polymerization reactors, or reaction of the Cr+6 species in the catalyst with the mineral oil itself could unfavorably alter the catalyst. Since one of the objects of the invention is improved catalyst feed control and commonality of equipment, the use of the same diluent for all catalyst families used in the polymerization reactor is highly desirable.
Furthermore, if slurrying the chrome-based catalysts in mineral oil would work, other opportunities to improve chrome-catalyzed resins would become feasible. One such possibility is to mix dissimilar but chemically compatible catalysts, such as, for example, supported silylchromate catalysts with chromium oxide catalysts before entering the polymerization reactor. Blending studies of resins made from both catalysts have suggested that various mixtures would exhibit improved product properties. For instance, a chromium oxide resin that is used for blow molding applications (for example, a polymer with a 0.953 g/cm3 density and 37 dg/min flow index (FI) has good stiffness and processability, but the environmental stress crack resistance (ESCR) is deficient. A resin produced a from silylchromate catalyst has excellent ESCR, largely due to its broadened molecular weight distribution (MWD), but has excessive bottle swell for many blow-molding applications. Feeding both catalysts separately from dry feeders is an option, however the ability to control both the absolute amount of each catalyst fed as well as the ratio of the two catalysts is extremely difficult using dry feeding techniques.
Static generation is also an area of concern for gas phase polymerization reactions. It is known that high levels of static are deleterious to continuous operation Static can be generated by a variety of means, including conveying of dry catalyst into the reactor. In practice, dry catalyst feeders inject catalyst at a high velocity into the fluidizing bed through an injection tube. This high velocity injection of a dry powder, particularly an insulating powder such as a silica supported catalyst, can conceivably generate static. One possible means to reduce static would be to use a liquid catalyst carrier to prevent charge generation. Another advantage then, of slurry feed of a chrome based catalyst to a gas phase reactor is the potential to reduce static in operation.
U.S. Pat. No. 5,922,818 claims a process to store a catalyst under an inert atmosphere, mix it in a hydrocarbon, and feed the suspension into a gas-phase polymerization reactor. U.S. Pat. No. 5,034,364 discusses the mixing of chromium catalysts by depositing both species (chromium oxide and silylchromate) onto the same substrate, but it does not mention use of separate supports for each catalyst or feeding a catalyst mixture to the reactor as a slurry. Two related patents, U.S. Pat. No. 5,198,400 and U.S. Pat. No. 5,310,834, discuss mixed chrome catalysts on separate supports but do not mention forming a slurry of a mixture. U.S. Pat. No. 5,169,816 discusses the deposition of various chrome species on an inorganic oxide support, preferably for use as a dry, free-flowing powder. WO 97/27,225 discusses mixing separate chromium catalyst species for polyethylene polymerization but does not disclose forming a slurry of the resulting mixture.
For purposes of United States patent practice, the contents of any patent or publication disclosed herein are incorporated by reference herein.